Nothing

Things need not have happened to be true. Tales and dreams are the shadow-truths that will endure when mere facts are dust and ashes, and forgot.

Dream, in Dream Country - "The Sandman" by Neil Gaiman

Everything.

People think dreams aren't real just because they aren't made of matter, of particles. Dreams are real. But they are made of viewpoints, of images, of memories and puns and lost hopes.